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In May 2013, the amount of carbon dioxide in Earth's atmosphere hit a previously unthinkable milestone.

For the first time in human history, the concentration of carbon dioxide in Earth's air reached 400 parts per million (ppm). The last time the planet experienced such high levels of the greenhouse gas was more than 3 million years ago, when conditions on Earth were unrecognizable from today and giant camels roamed a mostly ice-free High Arctic.

But what was once seen as an alarming threshold has now become business as usual. This year, scientists are forecasting that atmospheric concentrations of carbon dioxide will likely peak at around 417 ppm, signifying that for every 1 million molecules of gas in the atmosphere, 417 are carbon dioxide.

And with that, humanity will be charging into wholly uncharted territory.

"For millions of years, we haven't had an atmosphere with a chemical composition as it is right now,” said Martin Siegert, co-director of the Grantham Institute at Imperial College London.

The concentration of carbon dioxide is closely watched as an indicator of how humans are influencing Earth's climate. Burning fossil fuels releases carbon dioxide and other greenhouse gases that trap heat in the atmosphere. Elevated concentrations of carbon dioxide are associated with higher global temperatures, melting ice and rising seas, among other effects of climate change.

This year's rise in atmospheric carbon dioxide is expected to be 10 percent higher than normal, according to University of Exeter geography professor Richard Betts, head of the climate impacts division at the Meteorological Office, the U.K.'s national weather service. About 1 percent to 2 percent of the increase will come from Australia's devastating wildfire season, he added.

Australia's historic fires, which raged from September through early February, are thought to have unleashed about 900 million tons of carbon dioxide into the atmosphere.

When the planet last had an atmosphere that mirrored today's chemical makeup, Earth was in the midst of the Pliocene Epoch. During that geologic period, which lasted from about 5.3 million to 2.6 million years ago, humans had yet to appear on the planet, and average sea levels were up to 65 feet higher than they are today. Global average temperatures were also around 7 degrees Fahrenheit warmer, with temperatures at the poles likely double that, according to Siegert.

"There would have been a lot less ice on the planet — there probably wasn't a Greenland Ice Sheet, the West Antarctic Ice Sheet had probably melted, and big chunks of the East Antarctic Ice Sheet had probably de-glaciated, as well,” he said.

Projections have #CO2 shooting past 440 ppm in 2030. But...“It’s still under our control to a large degree. Future damage is still going to be controlled by future emissions so we still have an important control knob,” says Ralph Keeling. https://t.co/ZNveVf6CoF 1/2 pic.twitter.com/i4CWVZGAUl

Carbon dioxide levels have jumped about 100 ppm since 1958, when the American scientist Charles David Keeling first started recording daily atmospheric measurements using instruments at the Mauna Loa Observatory in Hawaii. To put that in perspective, a similar increase took 10,000 years, from the coldest part of the last ice age to its end, according to an unpublished working paper by Siegert and his colleagues at the Grantham Institute.

This year, according to forecasts from the U.K.'s Met Office, atmospheric carbon dioxide concentrations are expected to soar to 417 ppm. Siegert said this rapid rate of change is "deeply concerning.”

"We've done in a little more than 50 years what the Earth naturally took 10,000 years to do,” he said.

Carbon dioxide levels are typically represented on a graph known as the Keeling Curve, named for Keeling, who died in 2005. The iconic graph shows that carbon dioxide levels are increasing not only over time but also at a faster and faster pace in recent years. The sharp uptick, triggered by human-caused climate change, is apparent from the graph's steep incline, which was already apparent by the 1960s.

But within the graph's overall upward swing, the Keeling Curve appears as a jagged line, with each year registering a peak and a trough to match the natural swings in Earth's so-called carbon cycle.

The cycle is an intricate symphony in which the planet tries to regulate its temperature by naturally storing and exchanging carbon among the atmosphere, land, oceans, rocks and living organisms.

"In the spring and summer, as vegetation greens, this helps draw down carbon dioxide from the atmosphere,” Betts said. "In the fall and winter, carbon dioxide is released back into the atmosphere when plants die and trees shed leaves.”

That is why atmospheric carbon dioxide typically peaks in early spring, just before forest regrowth helps offset a part of greenhouse gas emissions.

But human-induced changes, such as burning fossil fuels or mass deforestation, can overwhelm the natural carbon cycle and outpace the planet's ability to offset elevated greenhouse gases.

Betts said the pace of change has been sobering to witness.

"The rate of rise in the last decade has been faster than previous decades,” he said. "We're just tracking ever onwards, and 400 ppm is now a distant memory.”

Ralph Keeling, a professor of geochemistry at the Scripps Institution of Oceanography — and the son of Charles David Keeling — said the only way to halt the trajectory of the Keeling Curve is to significantly cut fossil fuel emissions.

"As long as [carbon dioxide] keeps rising, each year's peak will exceed the peak from the previous year,” he told NBC News in an email. "We expect the peak this year to be just above 417 ppm. It looks like we are on pace to easily reach that high.”

But while the statistics may paint a bleak picture, Siegert said he is buoyed by the actions of young environmentalists who understand what's at stake in the climate crisis and are demanding change.

"They get it in a way that decision-makers right now don't,” he said, "so I'm optimistic, because I think, in the next 30 years, we'll be in good hands.”

Denise Chow

Denise Chow is a reporter for NBC News Science focused on the environment and space.